The eutrophication of lakes, rivers and other water resources is receiving worldwide attention. The presence in the environment of nutrients, such as phosphorus and nitrogen is one of the primary causes of eutrophication. These nutrients promote unwanted growth of algae and other aquatic plants which consume dissolved oxygen. In some instances, dissolved oxygen levels are reduced beyond the level needed to sustain fish and other animal life.
The eutrophication of our lakes and rivers has led to increased demands for nutrient control in the wastewater treatment plants. Governmental agencies have enacted increasingly stringent regulations controlling the amount of nutrients which can be discharged into receiving waters. Since conventional treatment processes remove only small amounts of nitrogen and phosphorus, wastewater treatment plants will be required to change or modify their processes to meet these increasingly stringent regulations. Unfortunately, the technology to achieve the required removal efficiencies is lagging behind regulatory requirements.
One approach for accomplishing nutrient removal is biological treatment in a modified activated sludge system without chemical addition. Numerous biological nutrient removal processes have been developed. These biological nutrient removal processes typically use a single sludge configuration in which the organic matter of the influent is used as the carbon and energy source for nitrogen and phosphorus removal. This allows for lower operating cost in comparison to multiple sludge systems and other physical-chemical systems.
One of these biological nutrient removal process which is commonly used is known as the Bardenpho Process. The Bardenpho Process consists of an initial anaerobic contact zone followed by four alternating stages of anoxic and aerobic conditions. In the anaerobic zone, all of the raw wastewater is mixed with the return sludge. The anaerobic conditions in the initial contact zone is necessary to effect phosphorus removal. The first anoxic zone follows the anaerobic zone. Nitrates and nitrites (No.sub.x) are supplied to the anoxic zone by recycling nitrified mixed liquor from the following aerobic zone. The organic material in the raw wastewater is used as a carbon source by the denitrifying bacteria in the denitrification zone. The first aerobic (oxic) zone is followed by a second anoxic zone where any remaining nitrites in the mixed liquor are reduced by the endogenous respiration of the activated sludge. The final stage is aerobic where the mixed liquor is reaerated before reaching the final clarifier. The dissolved oxygen of the wastewater effluent is increased to prevent further denitrification in the clarifier and to prevent the release of phosphates to the liquid in the clarifier.
The Bardenpho Process is capable of achieving a high percentage of nitrogen compound removal as well as phosphorus removal. However, the Bardenpho Process requires substantially larger tank volumes than conventional activated sludge systems which means higher capital outlays. Additionally, the Bardenpho System relies on endogenous respiration in the second anoxic reactor which is a relatively slow process. Thus, its use is limited to small plants.
Another biological nutrient removal process which is frequently used is known in the industry as the AAO Process (or A.sup.2 O Process). The A.sup.2 O process consists of three treatment zones--anaerobic, anoxic and aerobic. The wastewater and returned sludge are mixed in the first treatment zone which is maintained under anaerobic conditions to promote phosphorus removal. The anaerobic zone is followed by an anoxic denitrification zone. The third treatment zone is an aerobic zone where nitrification of the mixed liquor is achieved. The nitrified mixed liquor is recycled back to the anoxic denitrification zone where the nitrate and nitrite is reduced to elemental nitrogen by denitrifying organisms. The A.sup.2 O system has a high rate of nitrogen removal and requires total tank volume comparable to that of conventional activated sludge systems. Thus, the A.sup.2 O system is a cost effective system for nutrient removal. However, the A.sup.2 O system does not achieve high efficiency of nitrogen removal. The low nitrogen removal efficiency is an inherent limitation of the A.sup.2 O process. The maximum theoretical nitrogen removal efficiency can be calculated according to the following formula: ##EQU1## where: C.sub.NOx --concentration of NO.sub.3 +NO.sub.2 in plant effluent (g/l).
TN.sub.in --Concentration of Total Nitrogen in the influent (g/l). PA1 N.sub.B --Concentration of nitrogen removed by ordinary activated sludge process (The nitrogen removed by biomass to generate new cell material. PA1 IR--Mixed liquor internal recycle rate. PA1 Q--Influent flow rate. PA1 RS--Return sludge flow rate.
The equation assumes that there is complete nitrification in the aerobic zone and complete denitrification in the anoxic zone. Further, it is assumed that there is sufficient BOD available for complete denitrification.
In the A.sup.2 O system, the sludge recycle rate will typically equal 100% of the inflow, while the internal mixed liquor recycle will equal 200% of the influent flow. Using these values, the concentration of total nitrogen in the effluent would be approximately 1/4 of the total nitrogen in the influent. This correlates to a removal efficiency of approximately 75%.
According to the formula, the removal efficiency can be increased by increasing the mixed liquor recycle from the aerobic zone. If, for example, the mixed liquor recycle were increased to 400% of the influent flow, the concentration of total nitrogen in the effluent would equal 1/6 of the total nitrogen in the influent, for a removal efficiency of approximately 83%.
In actual practice, increasing the mixed liquor recycle in excess of 200% of the influent flow does not improve nitrogen removal. As the mixed liquor recycle increases, the recirculated mixed liquor dilutes the soluble BOD in the anoxic zone and thus decreases the rate of denitrification in the anoxic zone. The increased flow also decreases the actual retention time of mixed liquor in the anoxic zone and flushes out soluble BOD into the oxic zone where it is unavailable for denitrification.
Accordingly, there is a need for a biological nutrient removal process which accomplishes high nitrogen removal efficiencies at high reaction rates, which is cost effective, and which minimizes capital outlays required to retrofit conventional activated sludge systems.